The production of industrial-scale volumes of hydrogen is typically accomplished by application of the steam-methane reforming process, which entails the catalytic reforming of natural gas with steam at elevated temperatures (800-900° C.). This process yields a crude synthesis gas, which is a mixture of hydrogen, carbon monoxide, and carbon dioxide, and the crude synthesis gas is further reacted in a catalytic water-gas shift conversion step to convert carbon monoxide and water to additional hydrogen and carbon dioxide. The shifted synthesis gas is purified to yield a final hydrogen product containing greater than 99 vol % hydrogen.
The natural gas reforming reaction is highly endothermic, requiring about 45 kcal/mole of methane, and the productivity of the steam-methane reforming process is limited by the rate of heat transfer from the external heat source to the catalyst. The catalyst typically is contained in long metal alloy tubes, and the alloy is selected to withstand the elevated temperatures and pressures required by the process. A significant part of the capital cost of the steam-methane reforming process equipment is related to the need for significant heat transfer at the high operating temperatures and pressures.
An alternative process for the production of hydrogen is the partial oxidation of methane to form synthesis gas, which is subsequently shifted if necessary and purified by pressure swing adsorption (PSA). Partial oxidation is known to be highly exothermic. Another alternative process to generate synthesis gas for hydrogen production is autothermal reforming, which is essentially a thermally balanced combination of the steam-methane reforming process and partial oxidation. One considerable drawback associated with these alternative processes is that partial oxidation requires a supply of high purity oxygen gas to the reaction system. Therefore, the use of these processes requires the additional step of separating air to produce the oxygen gas, and the air separation process increases the capital and operating costs of hydrogen production.
Numerous methods for the production of hydrogen gas are known in the art. One method entails the reaction of metal oxides with steam and methane. United States Patent Application Publication No. 2002/0010220 describes the production of hydrogen and carbon monoxide by the partial oxidation and/or steam reforming of hydrocarbons in an autothermal process. The publication further discloses the use of an oxygen ion conducting, particulate ceramic in a cyclic process which involves the reaction of oxygen in the air feed with the ceramic in one step, and the reaction of hydrocarbon feed and, optionally, steam, with the oxygen-enriched ceramic produced in the first step, to produce hydrogen and carbon monoxide. Preferred ceramic materials are stated to include perovskite substances.
Similarly, the reaction of steam-methane using fluorite oxides is disclosed in “Hydrogen Production from Methane and Water by Lattice Oxygen Transfer with Ce0.70Zr0.25Tb0.05O2-x,” Z. C. Kang et al., J. Alloys and Compounds, 323-324 (2001) 97-101. Neither reference discloses the retention of carbon dioxide by the oxides to remove it from the hydrogen and carbon monoxide reaction products.
Investigations of the catalytic steam-methane reforming reaction have been carried out in systems which contain carbon dioxide acceptors to yield a higher-purity hydrogen rich product. For example, the use of calcium oxide, a carbon dioxide acceptor which is converted to calcium carbonate by chemisorption of the carbon dioxide, is disclosed in “The Process of Catalytic Steam-Reforming of Hydrocarbons in the Presence of Carbon Dioxide Acceptor,” A. R. Brun-Tsekhovoi et al., Hydrogen Energy Progress VII, Proceedings of the 7th World Hydrogen Energy Conference, Moscow, Vol. 2, pp. 885-900 (1988). The use of calcium oxide as a carbon dioxide acceptor in the steam-methane reforming reaction is also disclosed in “Hydrogen from Methane in a Single-Step Process,” B. Balasubramanian et al., Chem. Eng. Sci. 54 (1999), 3543-3552. Hydrotalcite-based carbon dioxide adsorbents are disclosed in “Adsorption-enhanced Steam-Methane Reforming,” Y. Ding et al., Chem. Eng. Sci. 55 (2000), 3929-3940.
U.S. Pat. No. 5,827,496 discloses a process for carrying out an endothermic reaction, such as the reforming petroleum hydrocarbons, within a packed bed reactor using an unmixed combustion catalytic material and a heat receiver. The catalytic materials are referred to as “mass-transfer catalysts,” and include metal/metal oxide combinations such as nickel/nickel oxide, silver/silver oxide, copper/copper oxide, cobalt/cobalt oxide, tungsten/tungsten oxide, manganese/manganese oxide, molybdenum/molybdenum oxide, strontium sulfide/strontium sulfate, barium sulfide/barium sulfate, and mixtures thereof. The heat receiver may also include a CO2 sorbent material, which is essentially limited to calcium oxide or a source thereof. This patent, in the context of its disclosed general process for heat transfer by “unmixed combustion,” describes a process for reforming petroleum hydrocarbons with steam.
U.S. Pat. No. 6,007,699 also discloses an “unmixed combustion” method that utilizes a combination of physical mixtures of metal oxides, a heat receiver and a catalyst comprising one or more metal/metal oxide combinations. Calcium oxide is used to remove carbon dioxide and drive the equilibrium reaction towards the production of hydrogen.
U.S. Pat. No. 6,682,838 discloses a method for converting hydrocarbon fuel to hydrogen-rich gas by reacting the hydrocarbon feed with steam in the presence of a reforming catalyst and a carbon dioxide fixing material, removing carbon monoxide from the hydrogen gas product by methanation or selective oxidation, and regenerating the carbon dioxide fixing material by heating it to at least 600° C. Suitable disclosed carbon dioxide fixing materials include calcium oxide, calcium hydroxide, strontium oxide, strontium hydroxide, and other mineral compounds containing Group II elements.
Known processes for the generation of hydrogen gas from hydrocarbons thus have associated drawbacks and limitations due to the highly endothermic nature of the hydrocarbon steam reforming reactions and the requirement of an oxygen supply for the partial oxidation of hydrocarbons used in autothermal reforming. There is a need in the field of hydrogen generation for improved process technology for the generation of hydrogen gas by the reaction of methane or other hydrocarbons with steam without certain of the limitations associated with known processes. This need is addressed by the embodiments of the present invention described below and defined by the claims that follow.